Hi Positive!
When I was little and growing up, I was always getting into trouble (tomboy trouble) and sprained my ankles, etc. I remember my mother would add epsom salts to my nightly bath when I hurt myself playing, oh yeah, I played hard...
Anyway, I believe it's safe to give your 3 year old detox baths.. Just not so much epsom salts.. And body warm temperature water... Below are a couple of links (there are probably many more articles but I haven't looked for them yet):
http://health.howstuffworks.com/skin-care/problems/treating/epsom-salt-baths3.htm
http://www.20somethingallergies.com/detox-bath-method-and-recipes/
Below is a link to an Australian site (you can google kg to pounds):
http://claybaths.com.au/professional/children.html
Hope this helps,
Denise